1. Technical Field
The present invention relates to an optical device and a detection apparatus.
2. Related Art
Recently, the demand for sensors used for medical diagnosis or inspection of food and drink has increased and development of a high-sensitivity and small-sized sensor has been required. In order to satisfy such requirements, various types of sensors using electrochemical techniques and the like have been studied. Among these, a sensor using surface plasmons resonance (SPR) has increasingly attracted attention from the viewpoints of possibility of integration, low cost, and usefulness for any measurement environment.
For example, JP-A-2000-356587 discloses a technique for improving the sensitivity of a sensor using localized surface plasmons resonance (LSPR).
In “Experimental Study of the Interaction between Localized and Propagating Surface Plasmons”, OPTICS LETTERS/Vol. 34, No. 3/Feb. 1, 2009, a technique of improving the sensitivity of a sensor using propagating surface plasmons (PSP) and localized surface plasmons (LSP) together is disclosed.
In JP-A-2000-356587, as shown in FIG. 1, metal fine particles 20 are fixed to the surface of a transparent substrate 10 and the transparent substrate 10 is irradiated with incident light to measure the absorbance of the metal fine particles 20. As shown in FIG. 2, when a target is attached to the metal fine particles 20, the absorbance spectrum indicated by A1 is changed to the absorbance spectrum indicated by A2. In the technique described in JP-A-2000-356587, the variation in medium around the metal fine particles is detected through the use of the change in absorbance and the adsorption or deposition of the target is detected.
However, in this technique, it is difficult to form the metal fine particles to be uniform in size or shape or to regularly arrange the metal fine particles. When the size or arrangement of the metal fine particles cannot be controlled, the absorption wavelength or the resonance wavelength caused from the plasmons resonance is non-uniform. Accordingly, as shown in FIG. 2, the width of the absorbance spectrum becomes larger and the peak intensity is lowered. When the peak intensity is lowered, the variation in the signal for detecting the variation in the medium around the metal fine particles becomes smaller and the improvement in sensitivity of a sensor is limited. Accordingly, the sensitivity of a sensor is not sufficient to specify a material in an absorbance spectrum.
In a surface-enhanced Raman scattering (SERS) sensor described in JP-A-2000-356587, since only one resonance peak is used, the wavelength of the resonance peak has to match any of an excitation wavelength or a Raman scattering wavelength. In this case, only an electric field enhancement effect in the scattering process of any one is used and thus a high electric field enhancement effect cannot be expected.
On the other hand, in “Experimental Study of the Interaction between Localized and Propagating Surface Plasmons”, OPTICS LETTERS/Vol. 34, No. 3/Feb. 1, 2009, as shown in FIG. 3, a sensor is disclosed in which an Au film 40 with a thickness of 100 nm is bonded onto a glass substrate 30, an SiO2 layer 50 with a thickness of 20 nm is formed on the Au film 40, and plural Au discs 60 with a diameter of 100 to 170 nm are two-dimensionally arranged on the SiO2 layer 50 in a period of P=780 nm.
In this sensor, the propagating surface plasmons (PSP) are excited in the interface between the Au film 40 and the SiO2 layer 50 and the localized surface plasmons (LSP) is excited in the Au discs 60. Here, the PSP has a specific wave number, and this wave number is determined by the dispersion relationship in the interface between the Au film 40 and the SiO2 layer 50 and the excitation wavelength. The wave number of the PSP is determined by the arrangement period P of the Au discs 60 and the real part thereof is equal to 2π/P. For example, when the excitation wavelength is selected from the visible range, the arrangement period P of the Au discs 60 is 780 nm, which is relatively large.
On the other hand, the Au discs 60 are sites (referred to as hot sites) having a large local electric field and an increase in sensitivity of the sensor requires an increase in density of the hot sites. However, the arrangement period P of the Au discs 60 used to determine the wave number of the PSP is 5 to 10 times the diameter of the Au discs, which is large, and thus the density of the hot sites is markedly small. However, when the arrangement period P is selected to be small, the coupling of the LSP and the PSP is weakened and thus a large local electric field cannot be achieved. When the outer size of the Au discs 60 is made to increase while guaranteeing a large period P, the resonance wavelength is shifted to a long wavelength side (red side) and departs from the excitation wavelength, and thus a large local electric field cannot be expected.
That is, in the structure disclosed in “Experimental Study of the Interaction between Localized and Propagating Surface Plasmons”, OPTICS LETTERS/Vol. 34, No. 3/Feb. 1, 2009, the hot sites having a large local electric field cannot be formed with a high density and thus a sensor having high sensitivity cannot be implemented.